CHOE SANG-HUN

SEOUL, South Korea — North Korea on Monday proposed talks with the United States to reach a formal peace treaty that would replace the truce that halted the Korean War 57 years ago, indicating that it would not give up its nuclear weapons until Washington signed such an accord.

North Korea said peace talks should be held either as part of the six-nation talks that focus on ending its nuclear weapons program or as a separate negotiation. But the North also warned that it would not return to six-nation talks — from which it withdrew last April — unless the United Nations lifted sanctions imposed after the North’s nuclear and ballistic missile tests last year.

The North had previously proposed peace negotiations with the United States and South Korea. But its latest overture came as it was trying to shift the focus of the six-nation talks, where a peace treaty had been set aside until North Korea made significant progress toward dismantling its nuclear weapons program.

“If a peace treaty is signed, it will help resolve hostile relations between North Korea and the United States and speed up the denuclearization of the Korean Peninsula,” the North Korean Foreign Ministry said in a statement carried by the North’s state-run news agency, K.C.N.A.

After years of unsuccessful talks with Washington, North Korea said it concluded that all agreements were bound to collapse unless the two sides built mutual “trust.” To build such confidence, the statement said, “It is essential to conclude a peace treaty for terminating the state of war, a root cause of the hostile relations.”

The statement reiterated North Korea’s contention that it would not have built nuclear weapons if the United States had assured it of peace.

Stephen W. Bosworth, President Obama’s special representative on North Korea, who visited the capital, Pyongyang, last month, said the United States could discuss a peace treaty and other incentives only when the process of denuclearizing the Korean Peninsula had gained “significant traction.”

Last week, Foreign Minister Yu Myung-hwan of South Korea denounced what he called the North’s “unrealistic” demand that the United States negotiate a peace treaty before the North considers relinquishing its nuclear weapons.

“That’s like saying it will never give up its nuclear programs, or it is a delaying tactic” to buy time to further its nuclear programs, he said.

On Monday, North Korea suggested that peace talks be held among the signatories of the Korean War armistice: the American-led United Nations Command in Seoul, China and North Korea. South Korea refused to sign the truce, but Seoul and Washington insist that any peace talks include the South.

Between 1997 and 1999, the two Koreas, the United States and China held six rounds of peace talks that produced no agreement because the North insisted on the withdrawal of American troops from South Korea and an end to joint United States-South Korean military exercises.

“With its peace proposal, North Korea is trying to gain the initiative as it prepares to return to six-nation talks,” said Kim Yong-hyun, a North Korea analyst at Dongguk University in Seoul. Mr. Kim expected some haggling between Pyongyang and Seoul over whether South Korea should be included.

Earlier Monday, Robert R. King, Mr. Obama’s envoy on North Korean human rights issues, said during a visit to Seoul that the North’s “appalling” human rights situation would impede any efforts to normalize ties.

He also called Monday for the release of Robert Park, a Korean-American missionary who crossed into North Korea late last month to demand the release of an estimated 160,000 political prisoners held in labor camps, according to his supporters in Seoul.

North Korea has confirmed that it has detained an American citizen but has not identified him by name.